--- a/dom/canvas/WebGL2ContextBuffers.cpp
+++ b/dom/canvas/WebGL2ContextBuffers.cpp
@@ -59,20 +59,28 @@ WebGL2Context::CopyBufferSubData(GLenum 
     };
 
     if (!fnValidateOffsetSize("read", readOffset, readBuffer) ||
         !fnValidateOffsetSize("write", writeOffset, writeBuffer))
     {
         return;
     }
 
-    if (readBuffer == writeBuffer &&
-        !ValidateDataRanges(readOffset, writeOffset, size, funcName))
-    {
-        return;
+    if (readBuffer == writeBuffer) {
+        MOZ_ASSERT((CheckedInt<WebGLsizeiptr>(readOffset) + size).isValid());
+        MOZ_ASSERT((CheckedInt<WebGLsizeiptr>(writeOffset) + size).isValid());
+
+        const bool separate = (readOffset + size < writeOffset ||
+                               writeOffset + size < readOffset);
+        if (!separate) {
+            ErrorInvalidValue("%s: ranges [readOffset, readOffset + size) and"
+                              " [writeOffset, writeOffset + size) overlap",
+                              funcName);
+            return;
+        }
     }
 
     const auto& readType = readBuffer->Content();
     const auto& writeType = writeBuffer->Content();
     MOZ_ASSERT(readType != WebGLBuffer::Kind::Undefined);
     MOZ_ASSERT(writeType != WebGLBuffer::Kind::Undefined);
     if (writeType != readType) {
         ErrorInvalidOperation("%s: Can't copy %s data to %s data.",

--- a/dom/canvas/WebGLContext.h
+++ b/dom/canvas/WebGLContext.h
@@ -1258,17 +1258,16 @@ protected:
     // Validation functions (implemented in WebGLContextValidate.cpp)
     bool InitAndValidateGL(FailureReason* const out_failReason);
 
     bool ValidateBlendEquationEnum(GLenum cap, const char* info);
     bool ValidateBlendFuncDstEnum(GLenum mode, const char* info);
     bool ValidateBlendFuncSrcEnum(GLenum mode, const char* info);
     bool ValidateBlendFuncEnumsCompatibility(GLenum sfactor, GLenum dfactor,
                                              const char* info);
-    bool ValidateDataRanges(WebGLintptr readOffset, WebGLintptr writeOffset, WebGLsizeiptr size, const char* info);
     bool ValidateComparisonEnum(GLenum target, const char* info);
     bool ValidateStencilOpEnum(GLenum action, const char* info);
     bool ValidateFaceEnum(GLenum face, const char* info);
     bool ValidateTexInputData(GLenum type, js::Scalar::Type jsArrayType,
                               WebGLTexImageFunc func, WebGLTexDimensions dims);
     bool ValidateDrawModeEnum(GLenum mode, const char* info);
     bool ValidateAttribIndex(GLuint index, const char* info);
     bool ValidateAttribPointer(bool integerMode, GLuint index, GLint size, GLenum type,

--- a/dom/canvas/WebGLContextValidate.cpp
+++ b/dom/canvas/WebGLContextValidate.cpp
@@ -117,37 +117,16 @@ WebGLContext::ValidateBlendFuncEnumsComp
         ErrorInvalidOperation("%s are mutually incompatible, see section 6.8 in"
                               " the WebGL 1.0 spec", info);
         return false;
     }
 
     return true;
 }
 
-/**
- * Check data ranges [readOffset, readOffset + size] and [writeOffset,
- * writeOffset + size] for overlap.
- *
- * It is assumed that offset and size have already been validated.
- */
-bool
-WebGLContext::ValidateDataRanges(WebGLintptr readOffset, WebGLintptr writeOffset, WebGLsizeiptr size, const char* info)
-{
-    MOZ_ASSERT((CheckedInt<WebGLsizeiptr>(readOffset) + size).isValid());
-    MOZ_ASSERT((CheckedInt<WebGLsizeiptr>(writeOffset) + size).isValid());
-
-    bool separate = (readOffset + size < writeOffset || writeOffset + size < readOffset);
-    if (!separate) {
-        ErrorInvalidValue("%s: ranges [readOffset, readOffset + size) and [writeOffset, "
-                          "writeOffset + size) overlap", info);
-    }
-
-    return separate;
-}
-
 bool
 WebGLContext::ValidateComparisonEnum(GLenum target, const char* info)
 {
     switch (target) {
     case LOCAL_GL_NEVER:
     case LOCAL_GL_LESS:
     case LOCAL_GL_LEQUAL:
     case LOCAL_GL_GREATER: